The invention relates to a method for the dynamic and density-independent determination of the mass flow of fluids and to devices for implementing this method. According to this invention, velocity and dynamic pressure of the fluid are determined at the same time and at the same place on one single vortex-generating bluff body placed in the flow and combined to give the resultant quantity "mass flow".
2. Prior Art
The conventional methods for determining the mass flow are normally based on measurements of the volumetric flow or of the velocity and the density of the medium and on the subsequent calculatory combination of these two measured values. Dynamic real time measurements of density are normally possible only in such media having chemical composition and thermodynamic functions of state which are known. This is, however, not the case in many technical applications. Even if the composition and the functions of state are known, the determination of density requires high measuring effort because it is necessary to measure pressure and temperature and to calculate the density from these data.
The devices used for various methods to determine the volumetric flow or the flow velocity possess movable parts, e.g., revolving vane aneometer and rotating lobe meter, and therefore are particularly susceptible to wear and contamination. Also methods involving pressure measurements, e.g., orifice measurements, and dynamic pressure measurement, which require transducer bores, pressure pipes and membranes, are often subject to contamination of the medium by solid particles or the like, which leads to substantial measuring errors or requires increased maintenance work for the measuring equipment. In addition, many of the conventional methods involve a high pressure loss due to the measuring device. Flow meters operating according to the principle of the Karman vortex path are already known (Vortex Shedding Meter, e.g., U.S. Pat. No. 3,587,312, German OS No. 28 27 985, U.S. Pat. No. 3,972,232, German OS No. 24 08 246, and German OS No. 20 37 198). These methods and devices are, however, only suited to measure the velocity and are not suitable for measuring the mass flow.
Approaches to determine the mass flow through the swirling of the fluid by bluff bodies are also known (e.g., the so-called swirl meter, U.S. Pat. No. 3,885,432 and German AS 14 98 271): by pressure measurement at the vortices, one single signal is obtained whose frequency is a measure of the velocity of the fluid and whose amplitude is a measure of the kinetic energy of the vortices. There is, however, no clear relationship which would permit conclusions to be drawn from the kinetic energy of the vortices on the kinetic energy of the fluid, particularly if the fluid is contaminated or contains constituents of different phases. In addition it should be noted that for an exact measurement the vortices must be well developed, which results in a high pressure loss.